Implementation of ultrahigh frequency emitters and applications to radar and telecommunications

ABSTRACT

The invention concerns an ultrahigh frequency emitting device, having: 
     at least a first and a second microlaser ( 22, 24 ), emitting at two different frequencies ω 1  and ω 2 , 
     means of slaving the first and the second microlaser frequency-wise, 
     an array of N elements (N≧2) ( 52, 54, 56, 58 ) placed on the path of the beam of the second laser, each element making it possible to impose a phase delay on the beam which passes through it, 
     N means ( 26, 28, 30, 32 ) for mixing the beam emitted by the first laser and each of the N delayed beams, and for producing N signals of frequency ω 1 -ω 2 , 
     N antenna-forming means ( 34, 36, 38, 40 ) for emitting radiation at the frequency ω 1 -ω 2 .

TECHNICAL FIELD AND PRIOR ART

The invention concerns the field of optical sources, and their use ascomponents in ultrahigh frequency emitter arrays.

Ultrahigh frequency emitters making use of optical sources are used inthe field of optical ultrahigh frequency telecommunications, asdescribed in the document by G. Grosskopf entitled “Optical fibresdeliver microwave broadcasts” published in O.L.E., Sep. 1996, p. 55-61.

One possible application of the invention is therefore “picocellularradiotelephony” with the use of the fiber telecommunication network asthe channel for transporting information to the radiating points.

The generation of ultrahigh frequency signals by optical signalheterodyning requires light sources of the laser source type, shiftedfrequency-wise by the value of the ultrahigh frequency sought.

This frequency shift can be obtained in different ways:

a) with two lasers, there can be obtained, if they have slightlydifferent optical cavity lengths, two emitters with slightly shiftedwavelengths, and therefore with emission frequencies capable ofdiffering by that of the ultrahigh frequency to be generated.

This principle of the optical transportation of ultrahigh frequencysignals is shown in FIG. 1.

Two optical sources 2, 4, emit respectively radiation at the frequency Ωand Ω+ω and are coupled to a mixer 6 (a photodiode) by means of opticalfibers 8, 10. The part 12, situated beyond the mixer, forms the antenna.A device 14 makes it possible also to slave the two lasersfrequency-wise.

b) Within a single laser cavity, it is possible to make two modes ofoscillation coexist simultaneously (with orthogonal, linear or circularpolarization states), whose frequency difference can be adjusted to thevalue of the ultrahigh frequency sought. Such a system is for exampledescribed in the document by M. Brunel et al. entitled “Differentialmeasurement of the coupling constant between laser eigenstates”published in Applied Physics Letters, vol. 70, no. 16, April 1997.

Other methods are known, like the use of an optical frequency shifter(typically, an acousto-optical modulator) for obtaining two sourcesshifted frequency-wise. This technique is not compatible with thefrequencies sought (≈10 GHz).

All the known devices pose a problem of size.

Moreover, in the known devices, correction devices are necessary onaccount of the over-large spectral width of the optical signal.

Finally, laser diodes, used in the majority of cases, are in generalmodulated frequency-wise by their supply current, and therefore also,simultaneously, emission amplitude-wise.

Another example field of application of the invention is the radarfield.

Modern radars use a so-called active antenna design in which the angularscanning function of the antenna is obtained not by rotation of theantenna itself but by that of its emission wave plane. The wave planeresults from a phasing—in the direction sought—of elementary wavesissuing from various radiating elements of the antenna. The phasing isgenerally obtained by the adjustment of delays on the transport routesof the different ultrahigh frequency signals. FIG. 2 shows thisprinciple schematically.

The radiation produced by an emitter 13 is divided into n beams eachpassing through means 15, 17, 19, 21 for imposing a delay on them. Thewave 23 emitted by the antennas 25 has its wave plane modified accordingto the different delays imposed.

The necessity of having a greater and greater number of radiatingantenna elements in order to improve, notably, the angular resolution(2000 emitters are spoken of), and the need to provide an accuratemanagement of the delays, lead to a complexity of the system which isdifficult to control with conventional ultrahigh frequency techniques(sizes and weights incompatible with the requirement of certain devices,airborne devices notably).

DESCRIPTION OF THE INVENTION

The present invention concerns the use of components in an array, inplanar and collective manufacturing technologies for implementingultrahigh frequency emitters.

The invention therefore concerns an ultrahigh frequency emitting devicehaving a number of lasers and N means making it possible to impose phasedelays on the path of N laser beams, these means being implemented in anarray or a bar.

The invention concerns in particular an ultrahigh frequency emittingdevice, having:

at least a first and a second laser, emitting at two differentfrequencies ω₁ and ω₂,

means of slaving the first and the second laser frequency-wise,

a mosaic or a bar or an array of N elements (N≧2) placed on the path ofthe beam of the second laser, each element making it possible to imposea phase delay on the bear or the portion of beam which passes throughit,

N means for mixing the beam emitted by the first laser and each of the Ndelayed beams, and for producing N signals of frequency ω₁-ω₂,

N antenna-forming means for emitting radiation at the frequency ω₁-ω₂,

The invention also concerns an ultrahigh frequency emitting device,having:

a plurality of N laser emitter pairs, implemented in a mosaic or anarray or a bar, each laser emitter pair having a first and a secondlaser emitter emitting at a first and a second frequency ω₁, ω₂, whichare different,

an array or a bar of N elements, each of them being placed on the pathof the second laser emitter of one of said laser emitter pairs, and eachelement making it possible to impose a chase delay on the beam of saidsecond laser emitter,

means of slaving each laser emitter pair, frequency-wise and phase-wise,

N means for mixing each of the beams emitted by the first emitters ofthe N laser emitter pairs with each of the beams emitted by the secondemitters of the N laser emitter pairs and delayed by the elements makingit possible to impose a phase delay, and for producing N signals at thefrequency ω₁-ω₂,

N antenna-forming means for emitting radiation at the frequency ω₁-ω₂,

The invention is based on a principle of ultrahigh frequencyelectromagnetic wave generation (a frequency which may reach severalhundred GHz) by means of the beating—the heterodyning—of at least 2electromagnetic waves in the optical domain (of much higher frequencies,of the order of 10¹⁴ Hz), generated by lasers. The use of elements inmosaics or arrays, for imposing phase delays, allows the implementationof a compact device.

The detection of frequency beats (a mixing function) is generallyprovided by a photodiode whose current is a non-linear function of theelectromagnetic field.

One of the advantages of the invention is the possibility of“transporting” ultrahigh frequency signals with a low attenuation perunit length by virtue of an optical “carrier”. The attenuation per unitlength in the fibers is in fact only of the order of 0.1 dB/km whereasit is 0.1 dB/m in an ultrahigh frequency conductor (coaxial).

According to another aspect, the laser sources can be microlasers orVCSELs (vertical cavity surface emitting lasers). These components arealso compatible with a collective implementation, in the form forexample of mosaics or arrays.

The device according to the invention then does not require any devicefor correcting the received signal.

This is because the chip laser sources (or microlasers) have a verysmall emission line width, of the order of a few hundred KHz, much lowerthan that of laser diodes (MHz) or that of VCSELs (also MHz).

Furthermore, chip lasers can be modulated (optical) emissionfrequency-wise with no “crossed amplitude modulation” (which is not thecase for laser diodes which are in general modulated frequency-wise bytheir supply current, and therefore also emission amplitude-wise) Thisfrequency modification is for example obtained by electro-optical typemodulation of the optical length of the microlaser cavity.

The invention also concerns an ultrahigh frequency emitting device,having:

a plurality of N laser emitter pairs, implemented in a mosaic or anarray or a bar, each laser emitter pair having a first and a secondlaser emitter emitting at a first and a second frequency ω₁, ω₂, whichare different,

means for slaving each laser emitter pair frequency-wise,

means for modifying the frequency of one of the laser emitters of atleast one laser emitter pair with resect to the frequency of the otherlaser emitter of said laser emitter pair,

N means for mixing each of the beams emitted by the first emitters ofthe N laser emitter pairs with each of the beams emitted by the secondemitters of the N laser emitter pairs and for producing a signal at thefrequency ω₁-ω₂,

N antenna-forming means for emitting radiation at the frequency ω₁-ω₂.

This device also has good compactness, on account of the structure ofthe laser emitters in a bar or a mosaic or an array. Moreover, the useof means for modifying the frequency of one of the laser emitters withrespect to the frequency of the other emitter makes it possible toobtain a slippage of the phase of one of the laser emitters with respectto the phase of the other laser emitter. It is then no longer necessaryto use specific means of varying the phases of the beams emitted by thelasers, as in the previous embodiments. Finally, the invention alsoconcerns a radar device having an ultrahigh frequency emitter asdescribed above.

The lasers or the laser emitters can be assembled in an array, acoupling or transmission by optical fibers being implemented between theelements making it possible to impose phase delays and the means formixing the emitted beams.

The frequency slaving means can also be assembled in an array.

Finally, the beat signal forming means can be merged with the means formixing either the beam emitted by the first laser and each of the Ndelayed beams, or each of the beams emitted by the first emitters of theN laser emitter pairs with each of the beams emitted by the secondemitters of the N laser emitter pairs and delayed by the elements makingit possible to impose a phase delay.

According to another aspect, the lasers or the laser emitters areassembled in an array and multiplexed by a multiplexer, an optical fiberconnecting the multiplexer and a demultiplexer.

For this, laser sources shifted optical frequency-wise, by themultiplexing step, can be implemented. To that end, an adjustment of thecavity length is implemented: each laser cavity has, for example,associated with it a Bragg grating type mirror implemented on acorresponding guide of the multiplexer.

The invention therefore also concerns an optical device having:

lasers or laser emitters (microlasers or laser diodes) implemented in amosaic or an array,

a multiplexing device having integrated optical guides, each opticalguide corresponding for example to a laser source or a laser emitter ora laser cavity,

a Bragg grating type mirror, implemented (for example: etched) on eachguide of the multiplexing device.

BRIEF DESCRIPTION OF THE FIGURES

In any case, the characteristics and advantages of the invention willemerge more clearly in the light of the description which follows. Thisdescription concerns the example embodiments, given by way of anon-limitative explanation, referring to accompanying drawings in which:

FIG. 1 illustrates the principle of the optical transportation ofultrahigh frequency signals with two lasers,

FIG. 2 depicts schematically the principle of an active antenna,

FIGS. 3A and 3B depict one embodiment of the invention,

FIG. 4 depicts an electro-optical device,

FIG. 5 depicts an array of phase shifters,

FIGS. 6A and 6B depict another embodiment of the invention, with anarray of emitter pairs, the associated phase modulation means also beingin an array structure, and the scheme for frequency slaving of theemitters,

FIG. 7 depicts means for the phase slaving of the signal of a deviceaccording to the invention,

FIG. 8A depicts an intra-cavity electro-optical modulator,

FIG. 8B depicts microlaser sources slaved phase difference-wise, byfrequency drift;

FIG. 9 depicts the frequency slaving of the device of FIG. 8B,

FIG. 10 depicts an active antenna radar operating using opticalheterodyning, with microlaser sources,

FIG. 11 depicts a microlaser cavity equipped with an electro-opticalmodulator and associated with an analyzer,

FIG. 12 depicts a device according to the invention, with wavelengthmultiplexing in order to reduce the number of optical fibre channels,

FIGS. 13A and 13B depict microlasers equipped with grating type mirrors.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A first embodiment of the invention will be described with relation toFIG. 3A.

Two lasers 22, 24 emit at two different frequencies: Ω and Ω+ω. On thepath of one of them there are placed means 52, 54, 56, 58 for generatingphase delays. The on laser beams are transmitted by fibers 42, 44, 46,48, 50 to mixers (optical photodetectors) 26, 28, 30, 32. The parts 34,36, 38, 40 situated beyond the mixers constitute the antennas.

The two laser sources 22, 24 are slaved frequency-wise. The slaving isdepicted schematically by the reference 25 in FIG. 3A. More precisely,it comprises (FIG. 3B) a photodiode 27 which receives a portion of eachof in the beams emitted by the lasers 22, 24 and which produces a beatsignal which serves as an input signal of a comparator 29. An ultrahighfrequency reference source 31 delivers its signal to the other input ofthe comparator 29. The latter produces an output signal used forcontrolling the frequency of the laser 24, for example by controlling anelement 33 for adjusting the optical length of the cavity of the laser24. Such an element is for example of the electro-optical ormagneto-optical type.

An electro-optical element 60 is depicted in FIG. 4. A voltage isapplied to it by means of electrodes 68, 70. The optical path of thelaser beam is modified by the application of a voltage between theelectrodes 68, 70. This is because there results therefrom a field E andan index ${n_{e_{L}} = {n_{e} - \frac{r_{33}n_{e}^{2}E}{2}}},$

where n_(e) is the index of the material 66 in the absence of any field,r₃₃ is the electro-optical coefficient, and E is the applied field.

There thus results therefrom a modification of the optical length of thecavity of the laser, and therefore of its emission frequency.

The electro-optical material 66 is, typically, LiNbO₃ or LiTaO₃ or anymaterial whatsoever having a significant variation of its index underthe action of an electric field (possibly a semiconductor).

The means 52, 54, 56, 58 are implemented in bar or array form. Thesemeans can be of the magneto-optical or electro-optical type. FIG. 5depicts four electro-optical elements 52, 54, 56, 58 placed on a support59 (transparent to the desired wavelength), on which they are, forexample, glued. The electro-optical materials are, furthermore,compatible with a collective implementation. Wafer manufacturingtechnologies make it possible to obtain simply, by sawing or polishingand sawing, the geometries of the modulators in bars or arrays on theirsupport 59: there is then no longer any need of gluing steps.

According to one particular embodiment, the invention uses “chip”lasers, or microlasers. The structure of microlasers will first bereviewed.

It is described, for example, in the article by J. L. Aubert, entitled“Q-switched microchip lasers bring new applications to light” publishedin “Laser Focus World”, June 1995.

The chip laser is a solid laser pumped by diode via, or not, an opticalfiber. All the elements constituting the laser (amplification medium,mirrors, modulators, etc.) are integrated in a small volume (<mm³) inorder to form a compact monolithic assembly. The laser mirrors are, forexample, directly affixed on the laser material, which is dielectric.

Conventionally, the laser material, constituting the active medium, isdoped with neodymium (Nd) for a laser emission around 1.06 μm. Thismaterial can be chosen, for example, from amongst one of the followingmaterials: YAG (Y₃Al₅O₁₂), LMA (LaMgAl₁₁O₁₉), YVO₄, YSO (Y₂SiO₅), YLF(YLiF₄) or GdVO₄, etc. It can also be a glass.

For emissions at other wavelengths, different dopants are chosen. Ingeneral, the active ions are chosen from amongst:

Nd for an emission around 1.06 μm (1.064 μm), and around 1.3 μm,

Er or an Er+Yb erbium-ytterbium co-doping for an emission around 1.5 μm,

Tm or Ho or a co-doping of thulium and holmium for an emission around 2μm.

Depending on the nature of the laser material and the doping, variousemission wavelengths are therefore possible. The two wavelengths 1.3 μmand 1.5 μm are advantageous for application to ultrahigh frequencytransport by optical routes, since they are situated within the windowsidentified for applications to optical telecommunications (availabilityof specific components assured by this market).

Various specific developments (stable cavities, frequency modulation,etc.) have been implemented around the basic structure of microlasers.

In particular, the document EP-653 824 describes a microlaser withpassive triggering for a saturable absorber. The document EP-724 316describes a monolithic solid microlaser with active triggering by a lowcontrol voltage.

The documents cited above, incorporated by reference in the presentdescription, also give methods of implementing microlasers. Thesemethods are collective and make it possible to implement manymicrolasers simultaneously.

In particular, microlaser bars or arrays can be implemented: for this,all that is required is to modify the cutting step in the collectiveimplementation method.

These microlasers can be incorporated in a device of the type describedabove with relation to FIG. 3A. The structure of the microlasers in anarray is compatible with the array structure of the means 52, 54, 56,58.

Also, instead of microlasers, VCSEL (vertical cavity surface emittinglaser) arrays can be used, which can themselves also give rise to anarray or bar implementation.

In all cases, the use of laser emitters in arrays or bars, incombination with the phase delay means, themselves implemented in arraysor bars, makes it possible to obtain an ultrahigh frequency emittingdevice which is very compact, and therefore compatible with theapplications (for example, “radar” type applications) where a very largenumber of emitters is required.

The phase modulator described previously is positioned “downstream” oftwo sources slaved frequency difference-wise. It can also beadvantageously used downstream of an assembly of a number of emitterpairs as far as the latter are slaved phase-wise (low-speed slaving,capable of being obtained in various ways: for example, thermally orelectro-optically, as described later). In this case, the topology ofthe system is that shown schematically in FIG. 6A, one of the two beamsof each pair of sources passing through a phase delay element 52, 54,56, 58. Each pair is slaved frequency-wise in the manner described abovewith relation to FIG. 3B, the ultrahigh frequency reference sourcepossibly being common to all the emitter pairs.

An array of emitters 61-2, 62-2, 63-2, 64-2 (the emitters 61-1, 62-1,63-1, 64-1 are not depicted) on a wafer, with associated phasemodulation means 52, 54, 56, 58, themselves also in an array structure,is illustrated in FIG. 7.

From a practical point of view, this topology has the advantage, byputting the sources in parallel, of providing a higher optical—orultrahigh frequency—power, while retaining an overall arrayconfiguration.

More precisely, the slaving function has, as illustrated in FIGS. 6A and6B, two levels:

1) a first frequency difference slaving 66, 68, 70, 72, of the opticalsources, fixed on an ultrahigh frequency reference source; this firstlevel of slaving has already been described above (FIG. 3B).

2) a second slaving, of the phase difference of the optical sourcesbetween one another, implemented by the use, for the differentmicrolaser pairs, of a common ultrahigh frequency reference source 31.

This ultrahigh frequency source 31 sends a common reference signal toall the microlaser pairs 62-1, 62-2, 63-1, 63-2, 64-1, 64-2.

The resulting signal can itself be controlled so as to allow modulationof the direction of the plane 23 of the emitted wave (see FIG. 2). Tothis end (FIG. 7), photodiodes 74, 76 take off the signals issuing, forexample, from neighboring modulator pairs 56, 58 or 52, 54 in order toform a beat signal from the microlaser pair beams. Each of these signalsis sent to means 78 which detect the frequency and phase difference withthe reference source 31 and correct this difference by voltage controlof each of the electro-optical modulators.

The same type of slaving can be used for the device of FIG. 3A. But itis less necessary since the phase rotation is then imposed by the twosources. Phase control can therefore be applied, in this case, with noslaving.

A variant of FIG. 6A would consist of placing a phase delay element foreach beam of each source pair, for example a phase delay of +φ/2 on thepath of the beams issuing from the sources 61-1, 62-1, 63-1 and 64-1 anda delay of −φ/2 on the paths of the beams issuing from the sources 61-2,62-2, 63-2, 64-2. More generally, a symmetrical and differential phaseshift can be implemented on the two routes.

In the embodiments described above, the sources are considered as fixedfrequency-wise and phase-wise (by slaving). The phase of the multipleultrahigh frequency emitters can be modified by making the relativefrequency of each of the emitter pairs slip (for a short time). Theadvantage, for phase modulation, of this frequency drift approach liesin the capability of modifying the phase in values much greater than+/−π (unlike direct modulation).

Optical frequency “drift” of the lasers can be obtained, as shownschematically in FIG. 8A, by modifying the optical length of theircavity. This modification of optical length is for example obtained byan electro-optical effect of the type of those already mentionedpreviously. The laser active medium 80 and the electro-optical material82 both form part of the microlaser cavity delimited by the cavitymirrors 83, 85. Here again the optical components for microlaser withmodulator are compatible with a collective implementation. Theapplication of a voltage between the electrodes 84, 86 brings about avariation in index of the electro-optical material 82, and therefore inthe optical length of the microlaser cavity.

There results therefrom a variation in the emission frequency of thelaser. In practice, the system has the overall structure depicted inFIG. 8B, a structure in which an electro-optical element on the path ofthe laser beams no longer appears: a phase relationship is imposedbetween two laser sources by making the frequency of one of the lasersslip until the desired phase difference is reached. For example, means88, 90 (to give examples) can be placed on each route (see FIG. 9) inorder to impose a phase delay of the ultrahigh frequency referencesignal. The “slippage” of the phase of the reference brings about aslippage of the frequency of the laser (since the reference signal ofthe corresponding comparator is modified), and therefore a slippage ofthe phase of one laser with respect to the other. The frequency slippagecontrol can for example be obtained by a phase-locked loop (PLL)controlled by an antenna orientation control center 91.

In FIG. 8B, the reference 65 designates a device providing a phasereference.

It is sought to reduce, as far as possible, the control voltage of thedevice 82 of FIG. 8A by bringing the electrodes 84, 86 closer together.In the case depicted (of a configuration for modulation at right anglesto the axis of the laser), the cross-section of the laser beam can thusbe decreased by the use of a so-called stable laser cavityconfiguration. A typical value of the modulation voltage—for the case ofcrystals—is then 10 MHz/V.

Each radiation source pair can advantageously be replaced by a dualfrequency source. The frequency-wise and phase-wise slaving is then thesame as that described above, but the device obtained is even morecompact and the slaving is less constrained since any drift on thecavity of the lasers has a second order effect on the frequencydifference of the double emitters. Dual frequency laser sources aredescribed in the article by M. Brunel et al. already cited above.

Furthermore, FIG. 8 gave the example of an electro-optical modulation. Asemiconductor modulation can also be implemented, with the advantage ofa modulation passband intrinsically higher than that obtained withelectro-optical crystals. Moreover, a planar and more compact geometryis then obtained.

Another application of the invention concerns radar systems.

The collective manufacturing technology for the phase modulators isadvantageous—for the radar application—through the array aspect of thestructure which this makes possible to obtain. Furthermore, thecollective manufacturing technology for the microlasers is alsoadvantageous in view of this application. This source structure (withphase modulation external or internal to the cavity) can be added to bysystem functions like, for example, those of slaving of the frequency,management of the phases with respect to a reference signal, orconnection to the optical fibres for transporting the ultrahighfrequency information. The end result is a system composed of thesuperposition of multiple “slices” with an optical or electronicfunction, which are extremely compact despite the complexity aimed at(slaving of several thousand ultrahigh frequency emitters).

FIG. 10 depicts schematically a radar system based on the opticalultrahigh frequency routes being implemented in array form. A variantwith bars can be implemented to optimize the manufacturing costs.

The device depicted has means 92 for coupling fibers 94 conveyingpumping beams issuing from an array of pumping diodes (not depicted inFIG. 10). The pumping beams are next directed to an array 96 of chiplasers (or microlasers or VCSELs) provided with an array 98 of phaseand/or amplitude modulation means. The reference 100 designates an arrayof means for acting on the phase and/or amplitude slaving of the chiplasers. The phase slaving means are for example means of the type ofthose already described above.

In particular, phase and/or amplitude set points are regulated by beatsignals coming from diodes which can be situated either on the path ofthe laser beams (upstream of the optical fibers) or at the end of thechain, the slaving diodes then being merged with the antenna diodes ofthe array 104. This second possibility makes it possible to take intoaccount phase delays connected with the optical fibers 102. The beatsignal is, in all cases, sent back to the slaving means of the array100. In the second case, it can be sent back by the fibers 102 or byseparate optical fibers. The optical fibers 102 make it possible totransport the modulated beams to an array 104 of photodiodes, which havea beam mixing function. These are means of detecting beat signals with aview to phase and/or amplitude slaving. The reference 106 designates anarray of active antennas provided with a suitable electronicamplification.

Amplitude adjustment of the different emitters can be implemented bymodulation of the pump power of the lasers. The modulation frequencyperformance is however then limited (<a few 100 kHz) for reasonsinherent in the laser material (level life time, in particular).

Amplitude modulation of the sources is therefore preferably implementedby electro-optical modulation. A description of this type of modulationis given in the work by A. Orszag and G. Heppner entitled “Lasers andtheir applications”, Masson, p. 1450, 1980.

As illustrated in FIG. 11, an electro-optical modulator is based on thechange in polarization direction of a light wave upon passing through ananisotropic medium 108, as a function of an external electrical fieldapplied by means of electrodes 110, 112. The effect of this change inpolarization direction—analyzed by means of a linear polarizer 114—is anamplitude modulation of the optical signal. Preferably, the analyzer 114is assembled on the electro-optical material 108.

On the assumption that the laser wave is not perfectly polarized onentry to the modulator, a polarizer (analogous to that of the analyzer)can be introduced upstream of the modulator.

Consequently, FIG. 10 describes the overall structure of an activeantenna radar operating using optical heterodyning. The phase and/oramplitude modulation or control means can be situated upstream of theoptical fiber transportation of the ultrahigh frequency signals. Theycan also (this is the case depicted in FIG. 10) be positioned in adistributed fashion between the upstream and the downstream of the fibernetwork: upstream of the fibers, the means of acting on the phase andamplitude of the waves is then found; downstream, the means of detectingthe difference from the “slaving set point”, this set point, and thedifference from this set point, possibly being advantageouslytransmitted by the fibers network.

In the structure of the system described above in relation to FIG. 10,there appear as many optical fibers as ultrahigh frequency routes to beimplemented (that is, for example, 2000 in an “objective”configuration). It is possible to reduce the number of these fibers bywavelength multiplexing (as performed in telecommunication withmultiplexers, of “Phasar” type for example).

For this, chip laser sources are implemented, shifted opticalfrequency-wise by the multiplexing step (typically 0.8 nm), byadjustment, for example, of their cavity length.

FIG. 12 shows this approach schematically with the implementation ofdual frequency chip laser sources: the microlaser 116 emits at thefrequencies Ω₁ and Ω₁+ω, the microlaser 118 at the frequencies Ω₂ andΩ₂+ω, etc. The reference 124 designates an optical multiplexer and thereference 126 a demultiplexer.

A phase reference can be obtained for each of the microlasers, asalready described previously. A means for providing a phase reference isindicated in FIG. 12 by the reference 131. Each of the demultiplexedbeams is next detected by a photodetector 128, 130, 132, 134.

From a practical point of view, adjustment of the lengths of thedifferent cavities can be implemented by a “distributed” cavity. Asillustrated in FIG. 13A, each laser cavity 136 has associated with it aBragg grating type mirror 138, advantageously implemented on thecorresponding guide (in integrated optics) of the multiplexing device.FIG. 13B depicts a microlaser assembly 140, the corresponding waveguides 142 and the grating 144 etched on the wave guides. Laser diodesare compatible with this scheme, as a replacement for the microlasers.But coupling of microlasers is easier to implement and, furthermore,microlasers are purer spectrally. Finally, dual frequency lasers canalso be used.

The invention therefore also concerns an optical device having:

lasers or laser emitters (microlasers or laser diodes) implemented in amosaic or an array,

a multiplexing device having integrated optical guides, each opticalguide corresponding for example to a laser source or a laser emitter ora laser cavity,

a Bragg grating type mirror, implemented (for example, etched) on eachguide of the multiplexing device.

What is claimed is:
 1. An ultrahigh frequency emitting device, having aplurality of laser emitter pairs, each of said laser emitter pairshaving a first and a second laser emitter emitting electromagnetic wavesin the optical domain, at a first and a second frequency ω₁, ω₂,respectively, ω₁ and ω₂ being different; a number of phase delayelements, each being placed in the path of said second laser emitter ofone of said laser emitter pairs, and each said phase delay elementadapted to impose a phase delay on the beam of said second laseremitter; means for slaving each said laser emitter pair in one of afrequency and phase manner, and a frequency, phase, and amplitudemanner; a number of means for mixing each of the beams emitted by saidfirst emitters with each of the beams emitted by said second emitters,and delayed by said phase delay elements thereby imposing a phase delay,and producing a number of signals at the ultrahigh frequency ω₁-ω₂ byheterodyning of said electromagnetic waves; means for converting opticalsignal to an RF signal; and a number of antenna-forming means foremitting radiation at the frequency ω₁-ω₂.
 2. The device of claim 1,wherein said laser emitters are microlasers.
 3. The device of claim 1,wherein said phase delay elements imposing a phase delay are selectedfrom the group consisting of electro-optical magneto-optical, andthermo-optical elements.
 4. The device of claim 1, wherein saidfrequency slaving means comprises means for forming a beat signal fromthe beams emitted by said first and second laser emitters of each saidlaser emitter pair, and means for adjusting the emission frequency ofone of said first and second laser emitters of said laser emitter pairaccording to the beat signal.
 5. The device of claim 4, wherein saidmeans for adjusting the emission frequency comprises means for comparingthe beat signal to a reference signal provided by a reference source,and means for modifying an optical length of a cavity of said one ofsaid first and second laser emitters for emission frequency adjustment.6. The device of claim 5, wherein said reference source is common to allof said laser emitter pairs.
 7. The device of claim 1, which furthercomprises means for slaving said phase delay according to a beat signalbetween the beam which passes through said phase delay element andanother beam.
 8. A radar device having an ultrahigh frequency emittingdevice as in claim 1, with said first and second laser emitters beingassembled in an array, a transmission by optical fibers beingimplemented between said phase delay elements, and means for mixing theemitted beams.
 9. The radar device of claim 8, wherein said frequencyslaving means is arranged in an array.
 10. The radar device of claim 8,wherein said frequency slaving means comprises means for forming a beatsignal from the beams emitted by said first and second laser emitter ofeach said laser emitter pair, and means for adjusting the emissionfrequency of one of said first and second laser emitters of said laseremitter pair according to the beat signal, said beat signal formingmeans being merged with said means for mixing one of the beam emitted bythe first laser emitter with each of the delayed beams, or each of thebeams emitted by said first laser emitters with each of the beamsemitted by said second laser emitters and delayed by the phase delayelements to impose a phase delay.
 11. A radar device having an ultrahighfrequency emitting device as in claim 1, with said first and secondlaser emitters being assembled in an array and multiplexed by amultiplexer, an optical fiber connecting the multiplexer and ademultiplexer.
 12. The device of claim 11, wherein each said first andsecond laser emitters has a cavity and wherein said cavities of saidfirst and second laser emitters are frequency shifted with respect toone another.
 13. The device of claim 12, wherein said cavities arefrequency shifted by a length adjustment.
 14. The device of claim 13,wherein each said cavity is associated with a Bragg grating type mirror,implemented on a corresponding guide of said multiplexer.
 15. The radardevice of claim 11, wherein said frequency slaving means is arranged inan array.
 16. The radar device of claim 11, wherein said frequencyslaving means comprises means for forming a beat signal from the beamsemitted by said first and second laser emitters of each said laseremitter pair, and means for adjusting the emission frequency of one ofsaid first and second laser emitters of said laser emitter pairaccording to the beat signal, said beat signal forming means beingmerged with said means for mixing arm of the beam emitted by the firstlaser emitter with each of the delayed beams, or each of the beamsemitted by said first laser emitters with each of the beams emitted bysaid second laser emitters and delayed by the phase delay elements toimpose a phase delay.
 17. A device according to claim 1 wherein thelaser emitters constitute an arrangement selected from the groupconsisting of a mosaic, an array and a bar.
 18. An ultrahigh frequencyemitting device, having a plurality of laser emitter pairs, each saidlaser emitter pair having a first and a second laser emitter emittingelectromagnetic waves in the electromagnetic domain, at a first and asecond frequency ω₁, ω₂, ω₁ and ω₂ being different; means for frequencyslaving each said laser emitter pair; means for modifying the frequencyof one of said laser emitter of at least one of said laser emitter pairswith respect to the frequency of the other laser emitter of said laseremitter pair; a number of means for mixing each of the beams emitted bysaid first emitters with each of the beams emitted by said secondemitters, and for producing a signal at the ultrahigh frequency ω₁-ω₂ byheterodyning of said electromagnetic waves; means for converting anoptical signal to an RF signal; a number of antenna-forming means foremitting radiation at the frequency ω₁-ω₂.
 19. The device of claim 18,wherein said laser emitters are microlasers.
 20. The device of claim 18,wherein said first and second laser emitters of each pair areconstituted by a dual frequency source, emitting at the respectivefrequencies ω₁ and ω₂.
 21. The device of claim 18, wherein said meansfor modifying the frequency comprises an electro-optical modulator. 22.The device of claim 21, wherein said electro-optical modulator is asemiconductor modulator.
 23. A device according to claim 18 wherein thelaser emitters constitute an arrangement selected from the groupconsisting of a mosaic, an array and a bar.